The present invention relates generally to variable speed drives. More specifically, the present invention relates to a system for precharging the DC link in a variable speed drive using insulated gate bipolar transistors in the rectifier or converter.
A variable speed drive (VSD) for heating, ventilation, air-conditioning and refrigeration (HVAC&R) applications typically includes a rectifier or converter, a DC link, and an inverter. The rectifier or converter converts the fixed line frequency, fixed line voltage AC power from an AC power source into DC power. The DC link filters the DC power from the converter and typically contains a large amount of electrical capacitance. Finally, the inverter is connected in parallel with the DC link and converts the DC power from the DC link into a variable frequency, variable voltage AC power. When electric power is applied to the VSD, the voltage across the DC link capacitors, referred to as the DC link voltage, rises from zero to a rated value, typically around 600 V. If this rise of the DC link voltage were left to occur naturally, it would happen very quickly by drawing very large electric currents from the input power lines, through the rectifier, and into the DC link capacitors. This large current, referred to as an inrush current, can be damaging to the components of the VSD. Thus, to avoid damage to the VSD components, the rise of the DC link voltage from 0 V to the rated voltage has to be accomplished in some controlled manner. This controlled raising of the DC link voltage is referred to as a DC link precharge operation.
Most VSDs accomplish a DC link precharge by two different methods. The first method employs precharge resistors and contactors connected between the input power line and the rectifier. The second method employs a rectifier consisting (at least partially) of thyristors, also called silicon controlled rectifiers, or SCRs.
In the first method, a precharge contactor is used to connect precharge resistors between the input power line and the rectifier or, sometimes, between the input power line and the DC link. These precharge resistors limit the inrush current to a manageable level. After the precharge is completed, the precharge resistors are excluded from the circuit by opening the precharge contactor, and the input power line is connected directly to the rectifier by closing another contactor, referred to as the supply contactor. The supply contactor remains closed during the operation of the system. This method is well suited for VSDs in which the rectifier is a simple diode rectifier, which offers no means for controlling the inrush current. The main disadvantage of this method is in the cost and size of its components, in particular of the supply contactor, which can negatively impact the cost and size of the entire VSD.
In the second method, the rectifier itself is used to accomplish precharge. The rectifier in this case has at least one SCR in each phase. SCRs are power semiconductors whose current conduction can be electronically controlled. The conduction of the rectifier's SCRs is controlled so as to let only small pulses of inrush current flow during precharge. After the precharge is completed, the rectifier's SCRs are controlled to conduct at all times, i.e., the rectifier after the precharge acts as if it were a diode rectifier.
The two precharge methods described above are applicable to VSDs whose rectifiers are made up of diodes and/or SCRs. However, there are VSDs with rectifiers or converters that do not use diodes or SCRs, but in fact, use insulated gate bipolar transistors (IGBTs) or other types of power switches or transistors. The IGBTs are usually packaged in modules and it is common for one module to include six IGBTs, which would be adequate for a three-phase rectifier. It is noted that the IGBT module can also be used for the inverter of the VSD. The typical IGBT module includes a diode for every IGBT present in the IGBT module, i.e., there would be six diodes in an IGBT module with six IGBTs. These diodes are commonly referred to, and connected, as anti-parallel diodes and are used to conduct current after an IGBT is turned off when the VSD operates in pulse width modulating (PWM) mode. The six anti-parallel diodes in the IGBT module can be considered to form a three-phase diode rectifier that is embedded within the IGBT module.
The embedded diode rectifier presents a problem for the precharge of VSDs that use IGBT modules because the first precharge method (precharge and supply contactors and resistors) described above must be used to precharge the DC link. This places additional cost and size burden on VSDs having IGBT modules for the rectifier or converter.
Therefore, what is needed is a system for precharging the DC link of a VSD having an IGBT-based rectifier or converter that does not require precharge and supply contactors and resistors.